1. Field of the Invention
Embodiments of the present invention generally relate to the deposition of thin films using chemical vapor deposition processing. More particularly, this invention relates to a process for depositing thin films onto large area substrates.
2. Description of the Related Art
Organic light emitting diode (OLED) displays have gained significant interest recently in display applications in view of their faster response times, larger viewing angles, higher contrast, lighter weight, lower power and amenability to flexible substrates, as compared to liquid crystal displays (LCD). After efficient electroluminescence (EL) was reported by C. W. Tang and S. A. Van Slyke in 1987, practical application of OLED is enabled by using two layers of organic materials sandwiched between two electrodes for emitting light. The two organic layers, in contrast to the old single organic layer, include one layer capable of monopolar (hole) transport and the other layer for electroluminescence and thus lower the required operating voltage for OLED display.
In addition to organic materials used in OLED, many polymer materials are also developed for small molecule, flexible organic light emitting diode (FOLED) and polymer light emitting diode (PLED) displays. Many of these organic and polymer materials are flexible for the fabrication of complex, multi-layer devices on a range of substrates, making them ideal for various transparent multi-color display applications, such as thin flat panel display (FPD), electrically pumped organic laser, and organic optical amplifier.
Over the years, layers in display devices have evolved into multiple layers with each layer serving different functions. FIG. 1 shows an example of an OLED device structure built on a substrate 101. After a transparent anode layer 102, such as an indium tin oxide (ITO) layer, is deposited on the substrate 101, a stack of organic layers are deposited on the anode layer 102. The organic layers could comprise a hole-injection layer 103, a hole-transport layer 104, an emissive layer 105, an electron-transport layer 106 and an electron injection layer 107. It should be noted that not all five layers of organic layers are needed to build an OLED cell. For example, in some cases, only a hole-transport layer 104 and an emissive layer 105 are needed. Following the organic layer deposition, a metallic cathode 108 is deposited on top of the stack of organic layers. When an appropriate voltage 110 (typically a few volts) is applied to the cell, the injected positive and negative charges recombine in the emissive layer to produce light 120 (electroluminescence). The structure of the organic layers and the choice of anode and cathode are designed to maximize the recombination process in the emissive layer, thus maximizing the light output from the OLED devices.
The lifetime of display devices can be limited, characterized by a decrease in EL efficiency and an increase in drive voltage, due to the degradation of organic or polymer materials, the formation of non-emissive dark spots, and crystallization of the organic layers at high temperature of about 55° C. or higher, e.g., crystallization of the hole transport materials can occur at room temperature. Therefore, a low temperature deposition process for these materials, such as at about 100° C. or lower is needed. In addition, the main reason for the material degradation and dark spot problems is moisture and oxygen ingress. For example, exposure to humid atmospheres is found to induce the formation of crystalline structures of 8-hydroxyquinoline aluminum (Alq3), which is often used as the emissive layer, resulting in cathode delamination, and hence, creating non-emissive dark spots growing larger in time. In addition, exposure to air or oxygen may cause cathode oxidation and once organic material reacts with water or oxygen, the organic material is dead.
Currently, most display manufacturers use metal-can or glass-can materials as an encapsulation layer to protect organic materials in the device from water (H2O) or oxygen (O2) attack. FIG. 2 illustrates the conventional packaging of an OLED device 200 on a substrate 201 with glass or metal encapsulating materials 205. The device 200 includes an anode layer 202 and a cathode layer 204 with multiple layers of organic materials 203. The metal or glass materials 205 are attached to the substrate 201 like a lid using a bead of UV-cured epoxy 206. However, moisture can easily penetrate through the epoxy 206 and damage the device 200.
Other materials, such as inorganic materials, e.g., silicon nitride (SiN), silicon oxynitride (SiON) and silicon oxide (SiO), prepared by plasma enhanced chemical vapor deposition (PECVD), can also be used as an effective encapsulation/barrier layer against moisture, air and corrosive ions for such devices. However, it is very difficult to generate water-barrier inorganic encapsulation materials using a low temperature deposition process because the resulting film is less dense and has high defect pinhole structures. It is important to note that the presence of residual moisture in the organic layers may also promote the Alq3 crystallization process even in encapsulated devices. In addition, oxygen and humidity being trapped during encapsulation and infiltrating into the OLED device to be in contact with the cathode and organic materials generally result in dark spot formation, which is a frequent OLED destroying defect. Therefore, a good encapsulation/barrier film also requires low water vapor transmission rate (WVTR).
Additional problems with thin film inorganic silicon nitride (SiN) related materials as the encapsulation/barrier layer arise. If the encapsulating layer is thick to serve as a good oxygen and water barrier, it is usually hard, fragile, and too thick to adhere well to a substrate surface, resulting in cracking or peeling off the substrate surface, especially at high temperature and humidity stressed conditions. If the encapsulating layer is made thin to improve adhesion and thermal stability, it is not thick enough as a moisture barrier. Therefore, additional layers or other manipulation may be required.
Thus, there is still a need for methods of depositing low temperature encapsulation/barrier films onto large area substrates with improved water-barrier and thermal stress performance to protect the devices underneath.